1. Field of the Invention
This invention relates to magnetic-based mechanical translators, and more particularly to mechanical translators with ferrofluid bearings.
2. Description of the Related Art
Ferrofluids are dispersions of finely divided magnetic or magnetizable particles, generally ranging between about 30 and 150 Angstroms in size, dispersed in a liquid carrier. The magnetic particles are typically covered with surfactants or a dispersing agent. The surfactants assure a permanent distance between the magnetic particles to overcome the forces of attraction caused by Van der Waal forces and magnetic interaction, and also provide a chemical composition on the outer layer of the covered particles which is compatible with the liquid carrier and the chemicals in the surrounding environment. Ferrites and ferric oxides employed as magnet particles offer a number of physical and chemical properties to the ferrofluid, including saturation magnetization, viscosity, magnetic stability and chemical stability. Several types of ferrofluids are provided by Ferrotec (USA) Corporation of Nashua, N.H. A summary of patents related to the preparation of ferrofluids is provided in U.S. Pat. No. 6,056,889, while the use of ferrofluid bearings in a moving magnet electrical generator is discussed in copending patent application Ser. No. 10/770,957, entitled “Electrical Generator With Ferrofluid Bearings”, filed on the same day as the present invention by Jeffrey T. Cheung and Hao Xin, and also assigned to Innovative Technology Licensing, LLC, the assignee of the present invention. The contents of this copending application are hereby incorporated herein by reference.
A ferrofluid's frictional coefficient is roughly related to its viscosity (measured in centipoise (cp)), but not directly. For example, a ferrofluid with a viscosity of 300 cp has been found to have a static friction coefficient of about 0.015, the EFH1 ferrofluid from Ferrotec (USA) Corporation has a viscosity on the order of 6 cp and a static friction coefficient of about 0.002, but a water based ferrofluid with a viscosity of 5 cp has been found to have a static friction coefficient of about 0.01. The higher friction coefficient for the somewhat lower viscosity composition has been attributed to surface tension associated with a water based solvent.
Low friction systems can have other problems. For example, depositing a thin film over a substrate by vapor deposition is performed in a vacuum chamber by rotating and translating the substrate inside the chamber to achieve uniform deposition over a large area. The mechanism that controls this motion is cumbersome, consisting of gears, chains, and both rotary and translational motion vacuum feedthroughs. A simpler less expensive motion control for the substrates would be highly desirable
Traditional lubricants employed to assist movement of a load bearing mechanism on a substrate, while considerably reducing the frictional forces resisting motion, still involve a considerable amount of friction. For example, whereas the static coefficient of friction between unlubricated hard steel surfaces is typically about 0.6, the corresponding coefficient is about 0.08-0.1 for vegetable and animal oil lubricants, 0.14-0.2 for mineral oils, 0.12 for graphitised oils, 0.1 for molybdenum disulfide, 0.08 for oleic acid, 0.4 for alcohol and benzene, and 0.2 for glycerine. Even lower frictional coefficients, making it easier to translate a load, would be desirable.